Acoustics 101: Sound + Vibration

02.15.2018by Hanson Hsu

Part 2 of Acoustics 101 for Architecture + Interiors: The Basics of Modern Day Acoustics: sound and vibration, timbre and materials science, traditional acoustics, absorbers, diffusors, quantizers, and quantum acoustics. Read Part 1.

What is Sound? What is Vibration?

Sound is energy riding on air molecules. Technically, it is the rarefaction and compression of air. Think of a slinky expanded and then compressed. Or waves in the ocean, which is energy riding on water molecules. It’s essentially all wave theory and physics. Energy can also ride on other gases, but as humans, we focus on air because it’s the most common medium in which we hear sound.

Vibration is sound, or more accurately, energy moving through a solid material like the ground or building materials like wood, steel, and drywall. Think of an earthquake. The timbre of the sound or vibration energy is affected dramatically by the behavior and character of the medium or material on which it travels, and in turn, that of the materials the medium or materials come into contact with. Simply put, the character of the air and the character of what the air touches greatly affects the timbre of sound and consequently the quality of the sound.

What is Timbre?

Per Merriam Webster, timbre is “the quality given to a sound by its overtones: such as the resonance by which the ear recognizes and identifies a voiced speech sound or the quality of tone distinctive of a particular singing voice or musical instrument.”

In modern acoustics, we use timbre in a broader sense, applied to all sound and not just to the voice or musical instruments. It’s more than frequency response or amplitude. It includes the character and obvious, identifiable traits of sound that give cues to the human ear. These traits are clear and distinct, creating emotional responses such as happiness or fear, elation or sadness, excitement or trepidation. Anyone who has laughed spontaneously, danced to music, or been barked at by an angry dog, completely understands this. While the basic elements of timbre are well defined, most of the more complex parameters are still to be determined and quantified.

When this research comes to fruition, it will forever change the future of acoustics. As with many things in nature, though we cannot yet fully understand them, when we hear sounds comprised of these cues and traits we always instantly identify them and the emotions they engender. This is perhaps the most inspiring and innovative topic in acoustic research: how to further define, measure, and quantify the psychoacoustics of timbre.

The Audible Character of Materials | Materials Science + Acoustics

Sound rides on air. As the air comes moves through an interior space, every material the air meets affects the timbre of the sound, much like how an electrical signal is subject to the characteristics of the copper cable through which it passes. Architecturally, every single material used to construct a room has a clearly audible, and often dramatic, effect on sound. Hence the importance of material science upon acoustics. How each material sounds and how it affects the air on which sound rides is important when specifying architectural materials.

Vibration is energy moving through solid materials. In architecture, the most common vibrational concerns are HVAC, airport, and roadway noise travelling through structural members. These noises typically becomes an issue when it translates into audible sound. Vibrations diaphragmatically move walls and ceilings, converting the vibrational energy into sound. This process is called transduction.

Just as the quality of the speaker and cables affect electrical signals, so does the quality of the studs, drywall, fasteners, and paint on the wall affect vibration.

Traditional acoustics is concerned with how the efficiency of transfer, frequency response, and amplitude/volume of the sound is affected by every single material in the chain of transduction. The impact of a material’s character on sound and vibration cannot be underestimated. The obvious solution to controlling sound and vibration is to control the materials in the room.

Consequently, materials science is an elemental part of acoustics, even more so since the discovery of Quantum Acoustics®. Materials science plays an enormous role in both traditional and quantum modalities, one rooted in ex post facto solutions based on classical mechanical behavior and the other in pre facto, elemental solutions based in the realm of quantum physics.

Absorption + Diffusion | Traditional Acoustics

Using absorptive materials and diffusers, Traditional Acoustics controls sound energy frequency by frequency and decibel by decibel (amplitude/volume). Absorption is a way to diminish the amplitude or volume of sound energy by trapping the sound energy in porous materials, which slow the sound down like a fast car trying to drive through a deep puddle. The car slows down as it hits the puddle and the energy transfers into the puddle itself.

Frequencies considered unpleasant or that stand out unevenly are remediated using these traditional techniques. Fiberglass batting such as Owens Corning 703 has been the most popular choice for many years, with newcomers like Thermafiber and Roxul being widely used in recent history. These absorptive materials are typically available in 2-inch to 4-inch thicknesses and a variety of densities, affecting specific sound frequencies based on both their density and thickness.

The most common misunderstanding is to use these absorptive materials as global, broadband solutions. They are very specific in their control of sound and not at all broadband controllers. Diffusors are typically wood panels, which are designed to evenly spread sound energy across an interior space. They look like panels made of complex slats or block arrangements and are used in conjunction with absorptive elements for best effect. Using these methods and more traditional acoustics seeks to control sound energy by identifying and then controlling problematic frequencies and amplitudes, remediating acoustical issues after the fact.

ZR Acoustics SR8

Quantization | Quantum Acoustics®

Using meta-materials and quantum physics applied to air molecules, Quantum Acoustics® controls air at the molecular level. As air is the medium on which sound travels, quantization removes the source of acoustical issues before they can exist by quantizing any air molecule before it contacts an interior surface. Air is forced to stop being a medium for the sound energy by forcing air molecules to behave as individual particles and not in groups as a wave. Hence, sound ceases to exist because it has no medium to travel on. Wherever applied, quantization is an acoustical cloak which removes the ability of the material surface beneath it to affect sound whatsoever. Quantum Acoustics® focuses on removing acoustical issues before they can exist rather than remediation after the fact. It is a new technology based on more than 100 years of applied quantum physics.

Part three of Acoustics for Architecture + Interiors will address typical applications of absorption, diffusion, and quantum devices using a specific example of solutions for both commercial and residential scenarios.